fix: pos exit proof non power of 2 merkle#908
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Polygon's matic-js MerkleTree (and the pos-contracts / pos-portal test helpers it mirrors) builds a complete fixed-depth binary tree by padding the leaf layer with zero hashes to the next power of 2. The on-chain verifier (pos-contracts/contracts/common/lib/Merkle.sol checkMembership) requires that exact shape — it asserts index < 2^proofHeight and walks a fixed-depth tree. merkleProof was instead duplicating the last node per layer when an odd count was reached. That coincidentally matches matic-js when the leaf count is already a power of 2 (mainnet's 128-block checkpoints), so the bug never surfaced in production. On a fast-cadence devnet validators sometimes submit checkpoint ranges that aren't powers of 2 (e.g. 40 blocks), and the resulting tree root differs from L1's stored headerRoot — startExitWithBurntTokens then reverts with WITHDRAW_BLOCK_NOT_A_PART_OF_SUBMITTED_HEADER. Pad once at the leaf layer to the next power of 2 with zero hashes, then build cleanly. Adds cmd_test.go with an independent reproduction of the on-chain verifier and a regression case at 40 leaves.
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jhkimqd
approved these changes
Apr 29, 2026
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Related to 0xPolygon/kurtosis-pos#545